There is no way to use Flash content on Firefox for Android. Adobe ended support for Flash on Android in 2013. Firefox supported Flash on Android up through version 56, that was several years ago.

Besides Adobe is discontinuing all support of their Flash Plugins for desktop at end of 2020 anyways.

Most websites have since changed to using HTML5 for video, audio, and games even which is also more cross platform compared to the more limited Flash.